In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoresponsive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet of the polygon and thereafter focused to a "spot" on the photosensitive member. The rotation of the polygon causes the spot to scan across the photoresponsive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoresponsive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction that is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster (or scan line) scanning pattern. The light beam is intensity-modulated in accordance with an input image data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent electrostatic image. The latent image is then developed and transferred to an appropriate image receiving medium such as paper.
Heretofore, a number of patents and publications have disclosed laser diode power control techniques, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 5,412,677 to Guerin et al., issued May 2, 1995, discloses a laser diode including a feedback system. The system compares pixel information against output power from a back photodiode to generate a feedback signal used to control current of the laser diode.
A difficulty in the past, is that other prior art techniques in power control of laser diodes have focused on thermo-electric (TE) coolers that regulate the temperature of the laser to minimize the power variation due to temperature. These (TE) coolers are expensive, bulky in size, and very inefficient to operate. More recent methods employ analog power controls which become increasingly difficult to implement when controlling the newest technology lasers with multiple diodes located within the same package. When using a multiple laser diode for simultaneous imaging, it is very important to balance the power of the beams to provide uniform exposure. This problem is exacerbated by the presence of thermal crosstalk between the plurality of diodes that may be placed within a common package. In addition, in prior art machines, exposure control has often been set by a "control knob" implemented with analog signal wires sensitive to noise.
Thus it would be desirable to provide a power system control that overcomes many of these difficulties in the prior art. It is therefore an object of the present invention to overcome not only changing characteristics due to temperature, but also differences between dual lasers in providing the necessary power balance. It is another object of the present invention to provide a microprocessor based digital control with embedded intelligence and diagnostic capability in controlling laser power. Another object of the present invention is to vary exposure setpoints by serial download of digital information including functional parameter data such as control loop compensation data. Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.
This invention is a power control method implemented by a microprocessor to digitally control the power from a multiple (preferably dual) laser diode within a Raster Output Scanner sub-system. The exposure power of each of two lasers is controlled for both the video ON exposure (Level) and the video OFF background (Bias) In particular, two different points on the laser diode characteristic curve are measured and each laser is controlled with two control loops, one for Bias and one for Level. The Bias control is done by indirect sensing method which also enables constant exposure power during Start Of Scan (SOS) detection. The Level control regulates the ON power for each of two beams to provide dual beam power balance with variable exposure as set by serial downloaded data.
In accordance with the present invention, there is provided a raster imaging system for exposing a photoresponsive surface moving relative to the raster imaging system in a process direction, including a multi-beam laser diode scanner forming a plurality of rasters across said surface, in a direction transverse to the process direction, by reflecting and modulating a plurality of beams so as to direct the beams to the photoresponsive surface; a controller to control the power from the dual laser diode, said controller including, means to control, individually, the exposure level from each of the plurality of laser diodes by controlling the laser diode current, wherein the laser diode current is a sum of a plurality of individually controllable input currents, comprising a prebias current, a bias level current, a video bias level current, and an exposure level current
In accordance with another aspect of the present invention, there is provided in a raster imaging system for exposing a photoresponsive surface moving relative to the raster imaging system in a process direction, a method for digitally controlling power from a laser diode comprising the steps of forming a plurality of rasters across said surface, in a direction transverse to the process direction, by reflecting and modulating a plurality of beams so as to direct the beams toward the photoresponsive surface; individually controlling the exposure power output by each of the multiple laser diodes by controlling the laser diode current, wherein the laser diode current is generated by summing individually controllable input currents, including a prebias current, a bias level current, a video bias level current, and an exposure level current.
One aspect of the invention is based on the discovery of techniques for controlling the bias and run power levels for multiple laser diodes so as to cause the beams to track on another in intensity. Such techniques can be implemented, for example, by a microcontroller or microprocessor to implement the feedback loop. The system described herein is advantageous because it is efficient and inexpensive compared to other approaches.